1. Field of the Invention
The invention relates to circuits which permit the connection of a telephone designed for non-party line use to a party line which selectively rings subscribers by the transmission from the telephone company central office of a ringing signal having a direct current component of either a positive or negative polarity referenced with respect to ground and a time varying signal for activating the telephone ringing mechanism.
2. Description of the Prior Art
FIG. 1 illustrates a prior art party line system of the type in which a telephone company central office 10 applies a DC component of a particular polarity referenced to ground to either the tip line 12 or ring line 14 of party line 16 and a ringing tone of conventional frequency, such as 20 Hz. The ringing signal is received by each of the subscribers to the party line 16 which are identified by the legends PTY 1 and PTY 2. Each subscriber location is provided with a gas discharge tube 18 having a pair of diodes having anode and cathode electrodes. The diodes are ionized into conduction upon the application of a potential of a magnitude above the breakdown potential of the tube. The gas discharge tube 18 associated with each subscriber is connected to the tip 12 and ring 14 lines such that the tube will break down only in response to the application of a DC potential of the correct polarity to either the tip or ring line to which the appropriately polled anode or cathode of the tube is connected. Breakdown does not occur with any other combination of DC polarity applied to the tip 12 or ring 14 line to which the tube is connected. When a negative DC potential above the breakdown potential of the gas discharge tube 18 is applied to the ring line 14, current flows from ground to ringing circuit 22 at subscriber location 20 consisting of a first inductor 24 in parallel with a series combination of a second inductor 26 and capacitor 28 through the ionized gas contained in the tube to the ring line 14 which causes the ringing circuit to produce ringing tone. The negative DC potential applied to the ring line 14 will not cause the gas discharge tube 18 associated with the subscriber location 30 to conduct for the reason that a negative potential is of the wrong polarity to break down the diode which is connected to the ring line. If a negative potential is applied to the tip line 12, current flows from ground through the ringing mechanism 32 at subscriber location 30 consisting of the parallel combination of inductor 34 and a series combination of inductor 36 and capacitor 38 through the ionized diode to the tip line 12 to cause ringing at the second subscriber location to occur. FIG. 1 does not illustrate the connection of third and fourth subscriber locations to the tip line 12 and ring line 14, but it should be understood that the third and fourth subscriber locations are connected in a manner analogous to the first location 20 and the second location 30, except that the third subscriber location has the electrode connections reversed from those illustrated at the first subscriber location 20 and the fourth subscriber location has the electrode connections reversed from those illustrated at subscriber location 30.
Other party line ringing systems which use DC signals of selected polarity to ring subscribers are disclosed in U.S. Pat. Nos. 582,107, 2,249,154, 2,532,125, 2,666,812, 2,875,281, 2,886,809 and 3,662,116. The systems disclosed in the aforementioned patents are not programmable to choose the combination of the polarity of the DC component and the line to which the DC component must be applied to activate the subscriber ringing mechanism.
Party line service of the type described in conjunction with FIG. 1 and in the aforementioned patents must have each subscriber's telephone location provided with a circuit which accepts only one combination of DC polarity applied to one of the ring or tip lines of the party line and a special telephone having a ground line for completing the ringing circuit current flow path between the party line central office and the subscriber's location. These prior art party line systems will not operate with a standard non-party line two-wire balanced telephone because of the absence of a ground line.